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Detecting a Possible Outbreak

Detecting an outbreak is the first step. An outbreak with hundreds of ill persons can be missed if they are spread out over a wide area.

One way health officials find outbreaks is through public health surveillance. By gathering reports of illnesses all the time, they know how many illnesses to expect in a given time period in a given area. If a larger number of people than expected appear to have the same illness in a given time period and area, it’s called a cluster. When an investigation shows that ill persons in a cluster have something in common to explain why they all got the same illness, the group of illnesses is called an outbreak. Detecting a cluster or possible outbreak of foodborne illnesses can occur in several ways.

Informal reports occur when members of a community call the local health department to report a group of suspected food-related illnesses. This might happen if, for example, several people got sick after eating at a group dinner.

Formal reporting systems play an important role in outbreak detection. Doctors and microbiologists in each state must report infections that are on a list of notifiable diseases when they diagnose them in patients. This list includes many foodborne illnesses. As public health officials look at disease reports, they may notice that the number of people with a particular illness is higher than expected. Sometimes an astute clinician realizes that he or she is seeing more cases of an illness than would be expected and calls the health department directly to discuss it with them.

For some pathogens, like the bacteria Salmonella and E. coli O157, public health laboratories do special tests to help detect clusters that might otherwise be missed. When a doctor suspects that a patient has a foodborne illness, he or she sometimes asks the patient to submit a stool sample (or some other type of sample). The doctor’s office sends the patient’s sample to a clinical laboratory. The clinical laboratory may isolate a certain bacteria and identify it as Salmonella or E. coli O157, for example. The clinical laboratory tells the doctor’s office what the patient has so the doctor can treat the illness, and then sends the bacteria to the state public health laboratory.

The state laboratory does further subtyping tests on the bacteria including serotyping and DNA fingerprinting or pulse-field gel electrophoresis (PFGE). Serotyping identifies the specific strain of bacteria based on markers on the surface of the bacteria. When several strains have the same markers or serotype all at the same time, and there are more with that one serotype than is expected, that’s a sign of a possible outbreak. DNA fingerprinting identifies the bacteria’s specific genetic pattern or DNA fingerprint. Bacteria can have thousands of different patterns. State laboratories report their DNA results to the PulseNet database.

Coordinated by CDC, PulseNet is the national molecular subtyping network for foodborne disease surveillance. By looking at the PulseNet database, health officials can identify clusters of illnesses caused by bacteria with the same fingerprint at the same time, even if the ill people are spread across many counties or states. This is especially useful when the number of illnesses in any one county or state is not big enough by itself to point to a possible outbreak. It can take 2 or 3 weeks from the day the person became ill to the day that the results of fingerprinting the bacteria are added to the PulseNet database. Please see the Timeline for Reporting of Salmonella Cases for more details.